An apparatus for thermal recording by means of thermosensitive paper and a transfer type thermosensitive recording media is commonly used in facsimiles and printers. In such a recording apparatus, a thermal head having heating elements arranged in a row is used as a recording head. However, the thermal head produces thermal energy during recording, and a problem results in that image quality may be deteriorated because of variations in the thermal energy. A primary reason for such image deterioration is voltage drop attributable to the so-called "black ratio".
The term "black ratio" refers to the ratio of the total number of heating elements in the thermal head being supplied with power at one time to the total number of heating elements in the terminal head. In the case of black-white recording, the percentage of black printing dots is determinative while, in the case of multicolor recording, the percentage of printing dots in each recording color is determinative.
As the black ratio fluctuates, voltage changes in the power supply itself or increased current flowing through the power supply line may cause a voltage drop. Accordingly, unless compensation is made therefor, the greater the black ratio becomes, the less the calorific value becomes of each heating element per unit time. In other words, the recording density is reduced proportionally and the desired gradation in contrast and/or color cannot be obtained. Where it is necessary to reproduce a delicate color using a plurality of recording colors, if the black ratio in the same recording portion differs for the various recording colors the desired recording density and the color balance may be adversely affected and the recorded image may be degraded.